Flash evaporation of pulp liquor to eliminate calcium salts



Nov. 9, 1965 J. E. Hol-:FT 3,216,788 FLASH EVAPORATION OF PULP LIQUOR TOELIMINATE CALCIUM SALTS Filed Allg. 24. 1961 MH. Sffmfffi AT TURN.' EY

United States Patent Oftce 3,216,788 Patented Nov. 9, 1965 3,216,788FLASH EVAPORATION OF PULP LIQUOR T ELIMINATE CALCIUM SALTS John E.Hoeft, Tomahawk, Wis., assignor to Owens-Illinois Glass Company, acorporation of Ohio Filed Aug. 24, 1961, Ser. No. 140,155 9 Claims. (Cl.23-49) This application is a continuation-in-part of my applicationSerial N-o. 658,994, tiled May 14, 1957 now abandoned.

This invention relates to the production of pulp. More specically, thisinvention relates to lso-called chemical processes for the production ofwood pulp, which pulp is ultimately converted to paper. Even morespeciically, this invention relates to a process of recovering residualchemicals utilized in the aforesaid chemical pulping operation.

Three broad classes of pulp manufacture are commonly utilized in theproduction of paper. The oldest type of process involves the separationof the iibers constituting the wood entirely by mechanical treatment,such as grinding or tearing. Another type of ber separation is effectedentirely by chemical digestion in which the wood chips are literallycooked in an aqueous solution of chemicals to soften lor remove theligneous or non-cellulosic encrustent and 'bonding materials which holdthe innumerable tiny cellulose fibers together, producing socalledchemical pulps. A third type of pulp production is accomplished partlyby chemical digestion and partly by mechanical treatment. The chemicalpulping operations known in the art 'include the soda pulping processutilizing an alkaline solution of sodium hydroxide, the sulfite processutilizing an acid solution of an alkali metal (sodium, potassium, etc.),sulfite, the sulfate or Kraft process, and the so-called semi-chemicalprocesses in which the cooking liquor includes a buiering agent toreduce the drastic acid or alkaline action.

It is economically desirable to recover the chemicals utilized in theabove referred to processes, and it is with this phase that thisinvetnion is m-ost directly concerned for an alkaline process. Therecovery of the chemicals necessitates at some stage of the process thestripping off of excess water, usually by evaporation, utilizing forreasons of thermal efficiency multiple effect evaporators.Unfortunately, however, it has been found that in the evaporation .stepa deposit 0f a salt or other complex organic material forms on the wallsof the tubes of the evaporator. This precipitation in the form of adeposit or coating seriously impedes the heat transfer within theevaporator whereby insucient water is removed, adversely affecting thelater `steps of the chemical recovery or, in fact, impeding the tlow ofthe process liquid therethrough.

Pri-or to the present invention, the removal of the deposited coating ina semi-chemical process operating on the alkaline side required theundesirable procedure of disconnecting the evaporators and soaking theinterior thereof with an acid solution. In more serious situations, thatis, where the coating was too thick to .be attacked by the acid, it wasnecessary to actually bore out the interior of the tubes.

Accordingly, it is an object of the present invention to provide a morecontinuously operable process for the recovery of chemicals fromchemical pulp produced |by an alkaline semi-chemical pulping process.

It is likewise an object of this invention to provide an improvement inthe process of recovering chemicals, which improvement eliminates thenecessity of frequent time-consuming and costly shut-downs for acidcleaning of evaporator components.

These and other objects of this invention will become apparent from thefollowing detailed description, taken in conjunction with the annexedsheet of drawings on which is presented for purposes of illustrationonly, a preferred embodiment of this invention.

In the drawings:

FIG. 1 is a flow diagram illustrating in partially schematic form .apulping process incorporating a chemical recovery process modied by theimprovement of this invention.

FIG. 2 is an enlarged sectional View of an evaporator utilized in thechemical recovery step.

Basically, this invention constitutes the discovery that precipitationin the form of a coating on the components of an evaporator may besubstantially eliminated by cooling the process liquid at the same orreduced pressure prior to evaporation.

Referring now more specifically to the drawings, there will be disclosedand explained in considerable detail the improvement representing thisinvention as incorporated into a specic semi-chemical pulping process inwhich sodium sulfite constitutes the pulping or digesting chemical, andsodium carbonate constitutes a buiiering agent.

Wood chips (about S in size) are fed at a rate of about 50,000 lbs. perhour into a pair of continuous digesters 11. Each digester consists ofsix 24" diameter, 20 long pipes (not shown), which are horizontallydisposed and stacked in vertical array. Each of the pipes contains aconveyor screw which propels the chips longitudinally along the 20length. Alternate ends of the pipes are connected in downward fashion tothe pipe lying beneath, thereby serving to propel the chips inalternatingly opposite directions through the pipes. Steam underpressure of lbs. per square inch gauge is also introduced into thereactor, effecting a temperature therein of about 375 F. The digestingor cooking liquor is contemporaneously fed into the digesters at Iatotal rate of about 80 gallons per minute. The liquor is essentially anaqueous solution of sodium sulte and sodium carbonate Iin the respectiveconcentrations of 0.294 lb. (expressed as NagO) and 0.233 lb. (expressedas NazO) per gallon of solution. The carbonate serves as a buiier foracids released during digestion. The pH within the digester measuresfrom 8.5 to 10.0. From the digester, the chemically attacked wood andthe spent digesting liquor pass to a deiibrator 12 which effects amechanical shearing and tearing of the chemically attacked wood chips.The mixture then passes to a blow tank 13 which is essentially a largeexpansion tower where the pulp is diluted from about 35% pulp solids(consistency) to about 4.5% pulp solids. The latter is conducted througha reiner 13a to prepare the fibers for washing. The pulp solution isthen conducted to a washing operation 14, where water is fedcountercurrent to the ilow of pulp and liquid, leaching out residualchemicals and various organic complexes produced during the digestingoperation. Vacuum drums are conveniently employed in this operation forseparating the pulp from the bulk of liquid, including spent digestingliquor. The separated pulp, measuring about 20% solids (consistency),passes thence to conventional pulp processing equipment includingrefiners 15, Fourdrinier paper-making machine 16, or the like, whichconverts the pulp into paper in the form of roll stock.

The filtrate from the pulp washing 14 passes to a temporary hold-up tank17. This filtrate is commonly referred to as black or spent liquor and,at this state, is at about F. It contains about 10% solids consistingprimarily of residual chemicals, but including various organiccomplexes, salts, and the like. Some of the iiltrate is recirculatedfrom the hold-up tank through the line 18 to the blow tank 13.Conventionally, the ltrate is next pumped at about 200 gallons perminute directly to a series of evaporators and 20a, one of which isshown as an enlarged section in FIG. 2. The liquid enters the evaporatorthrough an inlet tube 21 (FIG. 2) located in the bottom of theevaporator, and entering first a conical portion 22. When the conicalportion is filled with the liquid, the liquid then proceeds up through apluralityof upstanding tubes 23 leading through a heat exchangingchamber to an upper cylindrical chamber 24. Steam or vapor frompreceding effects enters a side inlet tube 25 and circulates about theoutside of the tubes 23, the condensate passing out through tube 26. Theheat converts some of the liquid passing through the tubes 23 into vaporor steam which passes out the chamber 24 through tube 27 which isconnected to a barometric condenser 2S (FIG. l), or to the nextevaporator via line 27a. The remainder of the liquid leaves the chamber24 through the tube 29 and passes to the succeeding evaporator. Forsimplicity of illustration, only two evaporators have been shown in FIG.1; however, a greater number may be conveniently employed in theinterest of thermal efficiency. As can be seen by referring to FIG. l,the steam from evaporator 20a passes to evaporator 20 and thence to thebarometric condenser 28. The process liquid being evaporated flows inreverse direction from the evaporator 20 to the evaporator 20a.

The number of evaporators may be chosen from a consideration of thethermal efficiency of the evaporation step and also should be sufficienteither in number or effective heat transfer area to convert the 10%solids solution filtrate to a solution containing about 45% to 50%solids. In the operation described herein, it was found that fiveevaporators, connected in series, were adequate to meet theserequirements when the last evaporator was fed with steam under 30 lbs.pressure.

The concentrated solution leaving the last evaporator is pumped at about4() to 50 gallons per minute to a recovery furnace 30. Fuel oil is alsofed to the furnace as a primary source of ignition. Some of the solidscontained in the solution fed to the furnace are combustible and theseare burned. The hot products of combustion are used to generate steambefore passing to the smoke stack 31. The non-combustibles comprisingessentially the chemicals utilized in the process are melted to formwhat is commonly referred to as a smelt. The smelt flows out the bottom32 of the furnace into a smelt dissolving tank 33 supplied with water.The smelt substantially dissolves in the water forming a green liquorconsisting primarily of sodium carbonate together with the lesseramounts of sodium sulfide and other salts. The green liquor so formedflows from the bottom of the dissolving tank and is circulated throughthe line 34. A tank 35 contains dissolved soda ash and supplies make-upsodium carbonate solution to the line 34 as required to bring theconcentra tion of the liquid up to the value specified hereinbefore. Theliquor then passes to a sulfiting tank 36 which is essentially a largeRaschig-ring packed a'bsorption tower into which is introducedconcurrently therewith sulfur dioxide gas proceeding from a sulfurburner 37 supplied by a source of sulfur 38. The amount of sulfur burntis controlled so as to provide sufficient sulfur dioxide to adjust theconcentration of the sodium sulfite in the liquor solution to the valuespecified hereinbefore. The liquid, as such, is referred to as sulfitedliquor or digester liquor and is reintroduced into the digester.

In the operation as just described, it is found that a precipitate inthe form of a coating is deposited on the inside of the tubes 23 of theevaporator 20. A similar deposit is formed on the interior of the tubesof the other evaporators as well. This deposit seriously reduces theeffective coefficient of heat transfer so that insufficient water isremoved. The deposit on the inside of the tubes also reduces thesectional area of the tube, increasing the difficulty of pumping. Whilethe exact nature of the deposit formed as a coating in the tubes is notknown and, While it is not desired to be bound yby any theory advancedherein, it is believed quite likely that the plrincipal component of theprecipitate or deposit is calcium oxalate. This would be likely formeddue to the presence of the calcium ion in the water, wood, and woodbark, as supplied, and the release of long chain fatty acids, includingoxalic acid, by the chemical attack of the ligneous component of thewood in the digestion step. Calcium carbonate is also believed containedin the deposit. The precipitate or deposit may well also consist of avariety of salts and organic complexes which would be likely toprecipitate under the conditions. In any event, in carrying out thechemical recovery operation as described herein, it was found necessaryto remove the deposit every four to six weeks to avoid loss of thermalefficiency. To do this, it is necessary to disconnect the evaporatorsand acid clean the interior of the tubes thereof with hydrochloric acidwash. The cleaning operation takes an appreciable period of time, e.g.,about ten hours and, in addition, consumes a sizable quantity of thecostly acid. In the improvement of the process constituting oneembodiment of this invention, the filtrate liquid is pumped at itstemperature of about 180 F. from the hold-up tank 17 to a hollow,upstanding, cylindrical flash tank 40, hav'- ing a conical bottomportion 41 serving to facilitate liow of the liquid therefrom. Theliquid is introduced into the tank through the side wall. The tank,which is otherwise sealed from the atmosphere, is connected by conduit43 connected at the top to a barometric condenser 28, serving to pull anexhaust, maintaining vacuum conditions Within the cylindrical tank. Thetemperature within the tank is maintained appreciably below that of theentering process liquid by permitting the hot, entering liquor to flashto the saturation temperature corresponding to the reduced pressure. Avacuum measuring 20 of mercury and a temperature of 162 F. has beenfound to effectively accomplish the purposes of the invention. The sizeof the tank should be such that the liquid entering at a rate of aboutgallons per minute will be freely projected into the interior of thetank. Employing the aforesaid flow rate, it has been found that a tank 9to 10 in diameter and having an overall height of about 17 to 20 issuitable. A head of liquid sufficient to intercept the entering streamis undesirable as, obviously, the stream would not then freely pass intothe atmosphere created by the establishment of the noted temperature andvacuum conditions. Rather, the liquid is allowed to immediately pass outthe bottom of the tank and is conveniently pumped via line 45 to theevaporator 20 for removal of water in the same manner as describedhereinbefore. A settling tank 44 is advantageously employed between theflash tank 49 and the first evaporator in order to allow hold-up andsettling of the process liquid following exposure to the zone of reducedpressure and temperature.

Carrying out the process as described, but modified as described in thepreceding paragraph, it was found that even after ten months ofessentially continuous operation of multiple effect evaporators, therehad not been. built-up any serious deposit of precipitate in the form'.of a coating on the evaporator tubes. As a consequence,`

no acid soaking and washing has been required. This.

elimination of the deposit or coating, through the prac-- tice of thisinvention, practically eliminates the amount:

of down time heretofore required to acid wash the evapo-v rators.Further savings in cost are also effected by thefact that the expensiveacid consumption is likewise avoided. Furthermore, the overall thermalefficiency of the operation has been increased thereby.

The foregoing descriptive material was clearly set forth in thespecification of such earlier application.

As described hereinabove, the coating or scale which previously formedon all effects of the multiple-effect evaporators exhibits compositionaldifferences in each effect. It was found in the feed or cold effect thatthe vsfcale consists essentially of calcium oxalate which is known to bemore soluble in hot water than in cold. The scale in the hot or rsteffect was observed to consist of essentially calcium sulte known to bemore soluble in cold water. In other words, the latter salt exhibits aninverse solubility. Scale collected from the intermediate stages of theevaporators contains mixtures of these prescribed salts as might beexpected.

In the preferred embodiment of the present invention set forthhereinabove, ash tank 40 is utilized primarily to cool the hot spentprocess of liquor from about 180 F. to a temperature more nearly equalto the temperature in the last effect of the evaporators, i.e., thattemperature being equivalent to 24" to 26 of mercury vacuum. Flashingthe hot spent process liquor to cool the same thereby causingprecipitation of detrimental deposits in a settling tank prior toevaporation has considerably improved the operation of the evaporators.

With regard to the theory involved in obtaining precipitation ofprimarily calcium salts by cooling, the following hypothesis has evolvedbased upon present knowledge. The cooking liquor contains a mixture ofsodium sulte, sodium carbonate, sodium sulde and minor quantities ofother compounds of sodium, sulfur and oxygen. This mixture forms anexcellent buffer solution that resists change in hydrogen ionconcentration upon the addition of acids or alkalies. The spent liquorsare believed to contain some of these chemicals in unchanged form aswell as other organic and inorganic compounds of sodium. These includesulfonated lignin, oxalic acid and sulfurous acid. Since the spentliquor is still alkaline, these acidic materials would exist in the formof salts and would be highly ionized. Analyses of the spent liquor haveindicated calcium contents much higher than possible in water. This maybe explained by the known dispersing action of sodium lignosulfonates.The spent process or black liquor provides an organic solution whichexhibits properties of having unusual solubilizing power for calcium inany form. It has been found with tube and shell-type heat exchangersthat if the hot spent liquor is cooled slightly, of the order of aboutF., precipitation of the calcium as a scale would form. It was foundthat if the calcium would precipitate on cooling in a heat exchanger, itshould also be expected to precipitate if cooled by any other mannerexcept by dilution with water. As described in the prior application themechanism of the deposition as deduced by reasoning was found to bevalid due to calcium salts being precipitated in the flash tank when thetemperature of the solution is lowered. It was also found that if thesolution was concentrated at the same time that it was cooled,sufficient calcium would be precipitated so that the carrying power ofthe liquor would be sufcient to eliminate the precipitation of scale inany effect of the evaporators. In other words, the calcium content ofthe liquor would then be reduced to such an extent that the solventaction of the liquor is then great enough to keep the remaining calciumin solution. The flash tank offers the advantages in that large amountsof crystals can be present to start crystallization and scaling in alocation of the operators choosing, i.e., a vessel having extensivesurfaces in which precipitated deposits are unobjectionable. Othermethods of cooling the spent liquor in a vessel or container maysimilarly be employed. In these cases the liquor is in contact withprecipitated seed crystals which produces the same beneficial results.Normal tube and shell heat exchangers are generally unsuitable for longterm continuous service because their relatively small diameter tubesbecome plugged. The liquor can be allowed to cool slowly in an open tankto obtain the same elTect, however, the volume of liquor in aconventional paper making process is so great that tremendous tankswould be required.

The spent liquor resembles a supersaturated solution and crystallizationcan be induced therein by relatively small changes in state such astemperature. In the case lof the particular spent liquor described, asudden relatively small change in temperature initiates thecrystallization and precipitation of calcium salts. Once started thecrystallization normally continues until the liquor constitutes no morethan a normal saturated solution of the remaining salt. The liquorbehaves in a manner similar to a colloidal suspension and the organicmaterials present act as stabilizing agents. As is known some colloidalsolutions or suspensions are sensitive to small changes in temperatureor pressure. The spent liquor behaves in such a manner as to indicatethat the quantity of calcium present is insuflicient to precipitate boththe salts of normal solubility and those of inverted solubility. Thus,if the solution is cooled the salt of normal solubility ispreferentially precipitated. This reaction reduces the calcium contentin the remaining liquor to the extent that the salt of invertedsolubility does not exceed the carrying power of the liquor even thoughit is concentrated several fold in the subsequent vacuum evaporators.

The present invention involves cooling the hot spent process liquor in avessel of such configuration and dimensions that the precipitation ofscale or deposits does not interfere with its normal function. Inaddition to the tiash tank, a cooling tank can be used but a largeindividual vessel or series of vessels are then needed. A largeheat-exchanger having mechanical means for removal of deposits can alsobe employed to cool the liquor. Also a spray tower which does not undulydilute the liquor can be used for cooling. Cooling by any of theaforesaid means does not result in subsequent fouling or plugging oftubes of relatively small diameter that are dicult or expensive toclean. The spent liquor represents a solution that is believed to be ina state of considerable instability. The fact that the solution is s0highly buffered rules out any appreciable change in pH as the result ofremoving acidic compounds in the flash tank or other cooling vessel.

Thus, as described hereinabove a substantial portion of the calciumcations are precipitated out of the spent process liquor prior toevaporation. The precipitation of the deposit-forming material ascalcium oxalate, calcium carbonate and calcium sulte is preferablyconducted in a vessel having suiciently extended surfaces 0n whichaccumulation of such deposits is unobjectionable. The spent processliquor is preferably cooled about 20 F. or more below the existingtemperature `of the liquor to effect precipitation. After suchprecipitation the remaining liquor contains insufiicient calcium cationsto react with sulte anions to cause adverse precipitation of calciumsulfte during subsequent thermal evaporation even to producing aconcentrated solution containing up to about 50% by weight total solids.

Various modifications may be resorted to within the spirit and scope ofthis invention, it being intended to be limited only to the extentexpressed in the appended claims.

I claim:

1. In the process 'of recovering chemicals from chemical pulp producedby an alkaline pulping process comprising separation of the pulp fromspent process liquor containi-ng a major portion of residual chemicalsincluding calcium cations and oxalate, carbonate and sullite anions,followed by evaporation of said process liquor to remove excess waterand thereby form a concentrated solution of said residual chemicals; theimprovement which comprises lowering the temperature of the hot spentprocess liquor at least 20 F., by injection into an evacuated flashingzone of essentially reduced pressure and temperature conditions, toeliminate the major portion of said calcium cations and oxalate andcarbonate anions by precipitation prior to concentration ofprecipitant-free liquor by evaporation.

2. The process in accordance with claim 1 including the step of loweringthe temperature of said process liquor through a sufiicient range toeiect precipitation `of essentially all calcium oxalate and calciumcarbonate.

3. In the process of recovering chemicals from chemical pulp produced byan alkaline pulping process comprising separation of pulp from aqueousspent process liquor containing a major portion of residual chemicalsincluding calcium cations and oxalate, carbonate and sulte anions,followed by evaporation of said process liquor to remove excess waterand thereby form a concentrated solution of said residual chemicals; themprovernent which comprises the steps of taking the said process liquorwhich contains loXalate and carbonate anions approaching saturation at atemperature of about 180 F. and cooling said process liquor at least 18to 20 F., by injection into an evacuated flashing zone of essentiallyreduced pressure and temperature conditions, to effect precipitation yofessentially all calcium oxalate and calcium carbonate` prior toevaporation of the remaining cooled process liquor.

4. In the process of recovering chemicals from chemical pulp produced byan alkaline pulping process comprising separation of pulp from aqueoushot spent process liquid containing a major portion of residualchemicals, followed by evaporation to remove excess water and therebyform a concentrated solution of `said residual chemicals, said processliquid containing calcium cations and oxalate, carbonate and sulteanions; the improvement which. comprises the intermediate step oflowering the temperature of the hot spent process liquid at least 20 F.,by injection into an evacuated dashing zone of essentially reducedpressure and temperature conditions, thereby precipitating calciumoXalate and calcium carbonate from said process liquid.

5. The method of concentrating the residual chemical content of aqueousspent alkaline pulping liquor prior to thermal concentration thereof byevaporation characterized by rapidly cooling the hot spent liquor byinjection into an evacuated flashing zone of essentially reducedpressure and temperature conditions, to effect precipitation ofessentially calcium salts, separating the precipitated calcium saltsfrom said liquor, and evaporating the remainring liquor containinginsufcient calcium cations to precipitate as calcium oxalate and suliiteduring subsequent thermal concentration.

6. The method of concentrating the residual chemical content of aqueoushot spent pulping liquor produced by an alkaline pulping process priorto thermal concentration thereof by evaporation characterized by theinjection of the hot spent process liquor into an evacuated ashing zoneof essentially reduced pressure and temperature conditions to effectprecipitation and separation of calcium salt constituents from theliquor prior to its concentration by evaporation.

7. The method in accordance with claim 6, further characterized by thesubjection of the said liquor to a pressure essentially less thanatmospheric in the said zone.

8. The method in accordance with claim 6, further characterized by thegravity separation of the effluent from the said zone into calcium saltand liquor cornponents and subjection of the liquor component only tothermal concentration.

9. The method of concentrating the residual chemical content of aqueoushot spent alkaline pulping liquor prior to thermal concentration thereofby evaporation including the steps of reducing the temperature of thehot spent liquor by injection into an evacuated flashing zone ofessentially reduced pressure and temperature conditions to eifectprecipitation of essentially calcium salts, the remaining liquor aftercooling containing insufficient calcium cations to exceed the solubilityof calcium sulfite at any stage of subsequent evaporation andconcentration up to about 50% total solids.

References Cited by the Examiner UNITED STATES PATENTS 1,809,427 6/31SpOhn 23-48 X 2,754,897 7/56 Ramen 23-48 X 2,755,168 7/56 COllen 23-48 XMAURICE A. BRINDISI, Primary Examiner.

GEORGE D. MITCHELL, Examiner.

6. THE METHOD OF CONCENTRATING THE RESIDUAL CHEMICAL CONTENT OF AQUEOUSHOT SPENT PULPING LIQUOR PREDUCED BY AN ALKALINE PULPING PROCESS PRIORTO THERMAL CONCENTRATION THEREOF BY EVAPORATION CHARACTERIZED BY THEINJECTION OF THE HOT SPENT PROCESS LIQUOR INTO AN EVACUATED FLASHINGZONE OF ESSENTIALLY REDUCED PRESSURE AND TEMPERATURE CONDITIONS TOEFFECT PRECIPITATION AND SEPARATION OF CALCIUM SALT CONSTITUENTS FROMTHE LIQUOR PRIOR TO ITS CONCENTRATION BY EVAPORATION.